The present invention relates to a method of making a ceramic cutting insert for metal machining and a ceramic cutting insert.
Ceramic cutting inserts are produced by powder metallurgical methods comprising milling, pressing and sintering. From the sintering process ceramic insert blanks are obtained. As a result of the pressing operation, the blanks have rather sharp cutting edges and also sinter skin as a result of the sintering process. However, sharp edges break too easily when used and sinter skin may contain surface defects acting as crack initiation points for an insert in operation. The sintered blanks must therefore be subjected to post-sintering treatment. Normally, there is also a need to perform a post-sintering treatment in order to obtain cutting inserts having the correct final shape and dimensional tolerances.
Ceramic insert materials for metal machining are of the hard but brittle type, thus easily obtaining surface defects, such as chipping, if surface treated in an inappropriate way. For this reason, care must be taken during the post-sintering treatment not to cause damage to the insert blank surface and particularly to the sharp, as-sintered, cutting edge. In some circumstances, such as when surface treating particular ceramic material grades, special measures must be taken in order not to cause unacceptable surface defects and hence rejection of the finished insert.
The common way of shaping ceramic insert blanks and removing the sinter skin is by using abrasive rubbing tools, such as conventional grinding wheels. However, grinding of ceramic blanks, in order to obtain the desired final shape, is known to be an expensive procedure requiring precision grinding equipment as well as extensive manual handling of the blanks or, alternatively, the use of expensive equipment for automated handling. In order to obtain an edge rounding of the cutting edge it is also necessary to perform a second post-sintering step, normally in the form of a brushing operation.
The resulting edge rounding can be described by a W/H ratio, where W is the width of the edge rounding along a rake face and H is the width of the edge rounding along a clearance face. In order to obtain the expected service life of the cutting insert, the W/H ratio achieved by the treatment must be within a certain tolerance.
The tumbling process has been used for many years for surface preparation and treatment of metal and metal alloy articles, for example for achieving deburring, radiusing or polishing of components for gas turbine engines, such as blades, vanes and nozzles.
U.S. Pat. No. 4,869,329 discloses a method of treating inserts for rock bits for drilling comprising a composite material of, e.g., tungsten carbide bonded with Co or a Ni—Fe composition, using extended vibratory tumbling in order to increase their fracture toughness.